86 THE NAUTILUS. 



Gills long and wide, the inner the wider, chiefly so anteriorly. 

 Outer gill narrowing gradually toward its anterior end, the 

 latter near the highest point of attachment line of mantle. 

 Inner gill narrowing anteriorly very suddenly, with its anterior 

 end about midway between the posterior base of palpi and ant- 

 erior end of outer gill. Outer lamina of outer gill entirely 

 connected with mantle ; inner lamina of inner gills free from 

 abdominal sac, except at anterior end. Behind the foot, the 

 two inner laminae of the inner gills are connected. 



The male has in both gills weak septa, distant from each other, 

 and incomplete (interrupted), and in the female, they are of 

 the same character in the inner gill. The whole outer gill of the 

 female is marsupial, and has distinct, heavy, and crowded septa, 

 running parallel to the gill- filaments, forming water-tubes. But 

 these septa are somewhat irregular and interrupted, chiefly to- 

 ward the margin, and in the posterior half of the gill, they are 

 rather regularly interrupted by transverse holes. The anterior 

 section of the gill has the most continuous septa. No heavy 

 mass of tissue is seen at the edge of this gill, which is sharp. 



Charged marsupium, embryos, and glochidia unknown. 



Color of soft parts (in alcohol) whitish, with black-brown 

 pigment on the edges of anal and branchial openings. 



That this species should belong in the affinity of Lampsilis 

 (or the subfamily Lampsilinae) , is out of the question. Th» 

 whole anatomy is truly that of the subfamily Unioninae of the 

 family Unionidae (Naut. 23, 1910, p. 116, and Ann. Cam. Mus. 

 8, 1912 p. 223, 224). The Anodontinae are excluded, since no 

 trace of a thickening of the edge of the marsupial gills is observed. 



However, a remarkable character of the septa of the marsupium is 

 that they are interrupted. This is an unusual character in the 

 family Unionidae, and more regularly found among the sub- 

 family Hyriinae oi the Mutelidae (1. c. p. 225). Yet there is 

 a Unionine form known, which has interrupted septa in the 

 marsupium, and this is Gonidea from the Pacific slope of North 

 America (see : Ortmann, Nautil. 30, 1916, p. 50). In Gonidea 

 all four gills are marsupial, while in Hyriopsis only the outer 

 ones are marsupial : but in their structure they are very closely 

 alike. 



